1 Field of the Invention
This invention relates to a magnetic resonance imaging signal generating system which, in response to frequency and phase control signals, provides first and second high frequency signals, one of which may be used to generate a magnetic field for rotating specific atomic nuclei and the other of which may be used for demodulating resultant magnetic resonant signals.
2 Background Information
A magnetic resonance imaging apparatus has the capacity of subjecting specific atomic nuclei in an object such as an living body to a magnetic resonance phenomena. This magnetic resonance phenomena results in the production of data relating to the spin density distribution and/or the relaxation time constant distribution of specific atomic nuclei within the object.
More particularly, in known prior art magnetic resonance imaging apparatus such as that disclosed in FIG. 1, an object 1 under observation is positioned within a composite magnetic field generated by both a static magnetic field generation coil and a gradient magnetic field generation coil, not shown in FIG. 1. After object 1 has been subjected to the requisite composite magnetic field, object 1 is thereafter subject to a high frequency excitation pulse which has a frequency corresponding to the resonant frequency of specific atomic nuclei within object 1. As shown in FIG. 1, this high frequency excitation pulse is represented by pulse signal f.sub.0. Pulse signal f.sub.0 is delivered to a head probe 2 which is disposed around object 1. A transmitter 3 is employed to generate pulse signal f.sub.0 in a manner well known to those skill in the magnetic resonance imaging art.
After high frequency pulse signal f.sub.0 supplied by transmitter 3 is delivered into probe head 2, a high frequency magnetic field results to create a magnetic resonance phenomena within object 1. This phenomena results in a generation of magnetic resonance imaging signals which are delivered from head probe 2 to a receiver 4, the output of which is transmitted to an amplifier 5 and then to a demodulator 6. The amplified signals are demodulated by demodulator 6 and delivered to a signal processor 7 in which the spin density distribution of the excited atomic nuclei, the relaxation time constant distribution of the excited atomic nuclei or the like are calculated for display.
The operation of transmitter 3 is governed in accordance with a first high frequency signal fr.sub.1 and the operation of demodulator 6 is governed in accordance with a second high frequency signal fr.sub.2. Signals fr.sub.1 and fr.sub.2 are generated by a signal generating device 8. The frequencies of both first and second high frequency signals fr.sub.1 and fr.sub.2 generally correspond to the resonant frequency of the atomic nuclei to be excited, typically the hydrogen atomic nuclei, within object 1. Accordinqly, the anqular frequency of high frequency signals fr.sub.1 and fr.sub.2 generally corresponds to the angular frequency W.sub.o determined in accordance with the well known Bohr's relationship: EQU W.sub.o =.gamma.H.sub.0
wherein:
H.sub.0 =the intensity of the static magnetic field to which object 1 is subjected; and PA1 .gamma.=the gyromagnetic ratio for the atomic nuclei under observation. PA1 m.sub.o (x,y,z)=the density of hydrogen atomic nuclei at a particular coordinate (x,y,z); PA1 T.sub.2 *=the spin-spin relaxation time of atomic nuclei influenced by the non-uniformity of the magnetic field; and PA1 .DELTA..theta.=the absolute value of phase differential between the angular frequency W.sub.o of the detected magnetic resonance signal and the angular frequency W.sub.r of the second high frequency signal fr.sub.2 used in connection with demodulator 6, so that .DELTA..theta.=.vertline.(W.sub.o =W.sub.r).vertline..
Since the resonant angular frequency W.sub.o of the hydrogen nuclei within object 1 is a function of the intensity of the static magnetic field and since the static magnetic field includes a gradient component, various slices of object 1 may be observed through frequency variation of first and/or second high frequency signals fr.sub.1 and fr.sub.2. For any given relation between the frequency of first signal fr.sub.1 and second signal fr.sub.2, a particular slice of object 1 may be observed.
In order to shorten acquisition time, a multi-slice method may be utilized in which the frequency of either the first or second high frequency signals fr.sub.1 and fr.sub.2 is offset by an amount .DELTA.f. In the alternative, a two-dimensional Fourier transforming method may be employed in which phase information is utilized as a significant detecting factor. In such signal acquisition processes, precise phase differentials .DELTA..theta. must be generated.
More specifically, if a magnetic resonance signal detected by receiver 4 has a resonant angular frequency W.sub.o and is demodulated by demodulator 6 using a standard second high frequency signal fr.sub.2 having an angular frequency W.sub.r, the resultant demodulated signal V.sub.f is represented by the following equation: EQU V.sub.f .alpha.m.sub.o (x,y,z).multidot.exp.sup.-(1/T.sbsp.2.sup.*+j.DELTA..theta.)
wherein:
It is critical that the phase differential between the received magnetic resonance signal and the demodulating signal be adjusted as precisely as possible in order to maximize the precision of the resulting image information. The phase differential .DELTA..theta. may be achieved by supplying the received magnetic resonance signals into delay lines which are comprised of inductance and capacitance circuits. However, some problems have existed in adjusting such circuits to the precision required and to assure that such delay lines are in an analog method. Moreover, if the intensity of the static magnetic field H.sub.o is increased in order to increase the performance of the magnetic resonance apparatus, the resonant frequency (lamor frequency) f.sub.o increases in proportion to the magnetic field intensity. Under these conditions, limitations appear in the capacity to correspondingly adjust the performance of the delay lines.
Furthermore, even if accurate control of the delay lines is feasible, the delay lines result in an increase in the size and cost of the resultant magnetic resonant imaging apparatus. Moreover, even if control of the phase differentials may be obtained through utilization of delay lines, when the resonant frequency changes with time as in the multi-slice method, it has been found almost impossible to perform the necessary phase adjustment with the requisite high degree of precision in the wake of these frequency changes.
It is, therefore, an object of the subject invention to provide a magnetic resonance imaging signal generating system which permits magnetic resonance signals to be measured with a high degree of precision by readily and precisely controlling the phase differential of the high frequency signals which are utilized to generate the magnetic resonance phenomena and/or signal demodulation of a magnetic resonance signal.
Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description or learned by practice of the invention.